1. Field of the invention
The invention relates to a process for processing expandable polymer particles, e.g. expandable polystyrene (EPS) particles. More particularly, the invention relates to a process for processing expandable polymer particles that contain an organic or an inorganic blowing agent or mixtures thereof in varying amounts. The particles are first over pressurized with gas; pre-expanded and dried; and then fed without maturing into a molding machine to form a foam article.
2. Background Art
Polymer foam articles molded from expandable polymer particles are well known. According to conventional practice, the conversion from expandable polymer particles to a molded foam article generally occurs in three steps: pre-expansion, maturing, and molding. The most commonly known and used expandable polymer particles are expandable polystyrene particles referred to as EPS. These expandable particles are generally made as solid, relatively “high-density” polystyrene beads of a relatively small size, e.g. beads having a diameter of from about 0.2 to 4.0 millimeters and a bulk (packing) density of about 600 kg/m3. Typically, these polystyrene beads are impregnated with a blowing agent, e.g. hydrocarbon, i.e. pentane, etc., and then heated with saturated steam to produce “pre-expanded” polystyrene particles, i.e. larger particles of lower density. If hydrocarbon is used as the blowing agent, the amount generally ranges from about 3.0% by weight to about 7.0% by weight based on the weight of the polystyrene. The blowing agent boils below the softening point of the polystyrene and causes the beads to expand when they are heated with the saturated steam, resulting in the particles being pre-expanded to a lower bulk density of about 8 to 80 kg/m3.
The pre-expanded particles, commonly referred to as “pre-puff” or “pre-foam”, must be aged for at least about an hour, and generally about 8 to 24 hours, to allow the particles to mature before they can be molded into a foam article. During the maturing process, air permeates the particles, and the internal pressure of the particles, which is initially lower than atmospheric, gradually approaches atmospheric pressure. At the same time, the external and the internal moisture in the particles evaporate. If pentane is used as the blowing agent, the condensation of pentane creates an under pressure compared to atmospheric pressure. During maturing, this condition is equilibrated and is necessary for good molding.
For the maturing process, it is known that there is a relatively narrow window (e.g. a few hours) of the optimum aging (maturing) time that will yield the highest quality moldings at the minimum molding cycle times. The optimum maturing time is determined by trial and error and is a complex function of such variables as bead size, blowing agent level, bulk density, pre-expansion conditions and maturing conditions (e.g. temperature, ventilation, etc.).
For the maturing process, the pre-expanded particles are generally stored in large vented bags. In general, a substantial amount of storage space is required, and scheduling and inventory control of the bags are cumbersome. In addition, the maturing process takes time. Also, if pentane is used as the blowing agent, some pentane vapor is released from the bags. It is difficult to capture and destroy this released pentane vapor since the air volume is very large and the relative pentane content is low. This presents an environmental and safety hazard, and, requires proper ventilation in order to avoid explosive air-hydrocarbon mixtures. A further disadvantage with the conventional practice of maturing or aging the pre-expanded particles prior to molding is the reduction in the flexibility of the plant. That is, the pre-expanded particles require that they be used within two to 24 hours after the maturing process. Therefore, the manpower and molding equipment in the plant need to be readily available at this time.
It would be desirable to have a process that does not require a maturing step whereby pre-expanded polymer particles may be molded immediately after the particles are pre-expanded.
There have been some developments that do not require storage and maturing of an intermediate pre-expanded product.
Collum et al., U.S. Pat. No. 5,271,886 assigned to Arco Chemical Technology, L.P. and issued on Dec. 21, 1993, uses a moving stream that contains carbon dioxide-impregnated polymer particles and a heating fluid. A pre-expansion unit provides a heated and thermally insulated expanding pathway through which carbon-dioxide impregnated polymer particles and the moving heating fluid flow. The moving stream of steam containing the pre-expanded polymer particles exits from the pre-expansion unit and is directed into a molding machine. The moving heating fluid is charged to the pre-expansion unit at a temperature from 250° to 500° F., and preferably, super-heated steam at a temperature from 350° to 500° F.
A further development that eliminates the maturing process for pre-expanded particles prior to the molding step is disclosed in A. P. August et al., U.S. Pat. No. 6,399,665 B1 issued on Jun. 4, 2002. This patent discloses that the pre-expansion of EPS particles has long been carried out with heat of condensation from steam that permeates the beads and deposits water droplets within expanding cells of the beads. This invention uses dry heated gas, such as air, to pre-expand the raw EPS beads through conduction only. The outer layer of the beads are first heated by the hot air and the heat penetrates conductively inwardly thereby forming a more pre-expanded structure (with thinner cell walls) on the peripheral surface of the beads and a less expanded structure (with thicker cell walls) at the interior of the beads. The pre-expanded beads contain a higher percent content of blowing agent since the blowing agent is locked within the interior cells of each bead. The resultant pre-expanded beads are dry and can be used immediately in molding of EPS foam articles due to excellent flow characteristics and excellent expansion capability (due to high residual pentane content). These resultant beads differ from those pre-expanded with steam in that thermal conduction enlarges peripheral cells more than interior cells in each bead, while steam acts by convection to permeate each bead (and condense within each cell), thereby expanding both peripheral and interior cells to substantially the same degree.
Detailed information explaining the technical reasons for maturing the impregnated particles, particularly those for conventional EPS containing pentane as the blowing agent, are given in the background section of the above-discussed U.S. Pat. No. 6,399,665. The maturing period generally allows the internal pentane pressure within the cells and the atmospheric pressure to reach a steady state equilibrium as well as to dry the pre-expanded beads sufficiently so that condensed water vapor on the surfaces of the beads does not cause the beads to agglomerate into lumps, which may not easily pass through the filling valve used in filling the mold or may not flow into corners and narrow spaces of the mold itself. This aging period also permits some of the water droplets (from condensed steam) that are inside the cells to escape through the cell walls thus drying the inside of the foamed beads. Without inside drying, the trapped water droplets sometime induce local non-uniformities, e.g. holes in the molded article because each droplet requires longer heating to vaporize it before heating and expansion of the surrounding cell can progress. However, care must be taken that the aging period is not too long as too much of the remaining blowing agent (e.g. pentane) may be lost by diffusion out of the cells of the pre-expanded beads, resulting in pre-expanded beads that no longer have the ability to expand further when they are heated during molding. When the beads do not expand sufficiently during molding, the molded foam articles tend to be poorly fused, and often crumble into pieces or leak their contents i.e. coffee in the case of coffee cups. Thus, for many years, proper aging of pre-expanded beads has been a delicate balance between a sufficiently long time needed to dry the condensed steam introduced during pre-expansion, and a sufficiently short time to retain an adequate amount of blowing agent, e.g. pentane within the pre-expanded beads. Aging not only allows some of the pentane and condensed water vapor to escape through the cell walls to the surrounding atmosphere, but also allows air to permeate back into the vacuum left inside the cells following expansion so that the particles do not collapse during molding.
As stated hereinabove, the inventions of U.S. Pat. Nos. 5,271,886 and 6,399,665 are directed to a process for processing expandable polymer particles that require no maturing or aging of the particles after the pre-expansion step and before the molding step. U.S. Pat. No. 5,271,886 pertains to particles that contain carbon dioxide as a blowing agent, and U.S. Pat. No. 6,399,665 pertains to particles that contain pentane as a blowing agent.
A further example of expandable polymer particles with carbon dioxide as a blowing agent is Meyer et al. U.S. Pat. No. 4,911,869. This patent teaches that because of the rapidity with which the carbon dioxide diffuses out of the polymer particles, it is necessary to first pre-expand the particles and then re-impregnate the particles with the same or a different gas just prior to molding. The above discussed U.S. Pat. No. 5,271,886 gets around this problem by expanding the beads to their final density as the beads are in route to the molding operation without maturing and without re-inflating the pre-puff prior to molding.
The use of carbon dioxide gas as a blowing agent in expandable polymer particles is also disclosed in Meyer et al. U.S. Pat. No. 5,049,328. This patent combines the steps of impregnation, purification, and foaming into a single process, which overcomes the rapid diffusion of the carbon dioxide gas out of the particle. Again, the above U.S. Pat. No. 5,271,886 gets around this problem by expanding the beads to their final density as the beads are in route to the molding operation without maturing and without re-inflating the pre-puff prior to molding.
There is still a need in the art to provide a process which eliminates the maturing step generally associated with conventional expandable polymer particles regardless of the type and/or amount of blowing agent in the particles after the pre-expansion process so that the pre-expanded particles can be immediately fed into the mold and “good” quality foam articles are formed.
There is a further need to provide a process that may eliminate re-impregnation prior to molding.